Speaking to reporters at his pre-match press conference ahead of Chelsea's match against Manchester United at Stamford Bridge on Sunday, the Blues manager revealed Bertrand needs first-team football.
The Chelsea official Twitter account said Mourinho hailed Bertrand as a "great player" but one that needed more time on the pitch to further his development.
The 24-year-old left-back has made just one Premier League appearance this term. Even when Ashley Cole was left out of the starting eleven earlier this term, Mourinho opted for right-back Cesar Azpilicueta in his place.
The Chelsea boss also confirmed winger Kevin de Bruyne is very close to leaving Stamford Bridge, while he was delighted with the capture of Nemanja Matic from Benfica - a deal completed earlier this week.
The loan spell will be Bertrand's sixth - he's previously spent time at Bournemouth, Oldham Athletic, Norwich City, Reading and Nottingham Forest.
He will be eligible to feature for Paul Lambert's Aston Villa side against Liverpool this weekend.